1. Field of the Invention
The present invention relates to a digital copier, printer, facsimile apparatus or similar image forming apparatus and, more particularly, to a writing device for use in an image forming apparatus for electrostatically forming a latent image on an image carrier with a light emitter array.
2. Discussion of the Background
A writing device using a light emitter array, particularly an LED (Light Emitting Diode) array, is extensively used in an image forming apparatus in order to reduce the overall size of the apparatus. In an image forming apparatus including the LED array, a photoconductive element in the form of a belt or a drum has its surface uniformly charged by a charger while in rotation. The LED array exposes the charged surface of the photoconductive element imagewise so as to electrostatically form a latent image thereon. After the latent image has been transformed to a corresponding toner image by a developing device, the toner image is transferred to a paper or similar recording medium. The LED array constituting the writing device has a plurality of e.g., seventy-eight LED chips arranged in an array and each having a plurality of, e.g., sixty-four LEDs or light emitters corresponding pixels. The number of LEDs of each LED chip is selected in accordance with a desired resolution.
Japanese Patent Laid-Open Publication Nos. 4-326854 and 4-326855 each discloses a bilevel image forming apparatus capable of reproducing desirable halftone images. The apparatus reduces a dot diameter for an image in which the distance between nearby dots is small, thereby preventing the image from being defaced. For an image in which the above distance is great, the apparatus increases the dot diameter in order to output a thick clear-cut image.
Specifically, the apparatuses taught in the above two documents both pertain to a bilevel image forming apparatus capable of rendering a halftone image by referencing observed pixel data and a plurality of pixel data surrounding the observed pixel on the same line, and variably adjusting the actual dot size for the observed pixel on the basis of the condition of such image data. The apparatus of Laid-Open Publication No. 4-326854 includes distance detecting means for determining distances between the observed pixel data and a print pit included in the surrounding data, and dot size control means for variably controlling the actual dot size of the observed image data on the basis of the above distance. On the other hand, the apparatus of Laid-Open Publication No. 4-326855 includes transition point detecting means for detecting transition points of the observed image data and surrounding image data from a print pit to a non-print pit or from a non-print pit to a print pit, counting means for counting the transition points, and dot size control means for variably controlling the actual dot size of the observed image data on the basis of the number of transition points.
Japanese Patent Laid-Open Publication No. 5-176131 discloses a scanning type image forming apparatus capable of preventing observed black data from being shifted and thereby implementing a dot diameter close to a theoretical single pixel. Specifically, the scanning type image forming apparatus includes an analog image signal generating section for generating an analog image signal. An analog-to-digital converting section transforms the analog image signal to a digital image signal. A bilevel processing section binarizes the digital image signal by use of a preselected reference level to thereby output a bilevel image signal. The reference level is output from a reference level generating section. The data of the binary image signal are written to a pixel data memory. A laser ON/OFF signal generator generates a laser ON/OFF signal for selectively turning on or turning off a laser. A laser signal modulating section modulates the laser ON/OFF signal with the bilevel image signal stored in the memory. A laser power switching signal generating section switches laser power for pixels around an observed pixel in order to adjust the density of the observed pixel. A laser power generating section causes the laser to emit in accordance with the output of the laser signal modulating section and laser power switching signal. A laser scanning section causes a laser beam to scan a photoconductive element so as to expose the element imagewise.
A problem with the conventional image forming apparatus or image recording apparatus is that when a photograph or similar halftone image is output, the image appears as if it were slightly lost in places. This is ascribable to some error in the distance between nearby LED chips although the LEDs of each LED chip are regularly arranged to implement a desired resolution. The resulting image therefore appears as if it were lost at the intervals between nearby LED chips. This occurrence is particularly true with a halftone image, as follows. In the case of a high density image, a light beam issuing from the individual LED has a great diameter. As a result, dots formed by the LEDs on the image carrier tend to overlap each other, rendering the local omission of the image inconspicuous. However, in the case of a low density image, the beam issuing from the individual LED has a small diameter with the result that the local omission ascribable to an error in the distance between the LED chips is conspicuous.
The local omission of an image will be obviated if the image data of the pixels corresponding to the interval between nearby LED chips are increased by a preselected amount or converted to a density rendering the local omission inconspicuous. However, if such image data are simply corrected, even the image data representative of white will be corrected and will thereby form thin vertical lines on a white background. In addition, even image data sufficiently high in density and not needing correction will be corrected.